Two-dimensional picture images in black and white, or more precisely in various shades of gray, are conventionally prepared for transmission over channels of restricted bandwidth by means of raster scanning. In raster scanning a moving beam of electrons or spot of light travels over the image mounted on a drum or flat-bed scanner in a regular pattern of lines sweeping from the top of a frame to the bottom. Typically, for a slow-scan analog facsimile system, such as is afforded by the use of the Western Electric Data Set Type 602A, a transmission time of about six minutes is required to send over the switched telephone network a sketch, order or message of an 81/2 by 11-inch letterhead size with a definition of 96 lines per inch at a speed of 180 lines per minute.
A digital system providing the same resolution for the approximate 71/2 by 10-inch sight area with an aspect ratio of three-to-four contains about 720 picture elements (pixels) per line and 960 lines per sheet for a total of 691,200 pixels. If there are sixteen possible gray levels assignable to each pixel, then four bits are required to transmit each pixel for a total of 2,764,800 bits. A Western Electric Data Set Type 208B operating at 4800 bits per second in the voice-frequency band of the switched telephone network thus requires 9.6 minutes to transmit an image which is the size of a business letterhead. What the digital system lacks in speed of transmission relative to an analog system is made up in reliability and adaptability to processing.
Analog facsimile scanning systems are well known in the art and may be found described in Chapter 2 starting at page 2-67 in Communication Systems Engineering Handbook edited by D. H. Hamsher (McGraw-Hill Book Company, New York 1967). Digital systems are disclosed in Digital Picture Processing by A. Rosenfeld and A. C. Kak, published by Academic Press in 1976.
It is straightforward to encode the gray-scale values of individual pixels as binary numbers to as many as 6, 8 or 10 digits as desired. An eight-bit number encodes, for example, 256 gray-scale levels, i.e., the digit two raised to the eighth power. Conversion of gray-scale values to digital numbers facilitates transmission of picture information over pulse-code modulation (PCM) channels.
In line-by-line transmission, whether transmission is by analog or digital means, stationary images are scanned as a sequence of rows, top row first and bottom row last. In order to view the entire image one must wait for substantially complete transmission. However, if the image information is reorganized at the transmitter terminal by means of high-speed apparatus, the low-speed transmission can furnish the gross structure of the entire image soon after the beginning of transmission and add increasingly detailed refinements as transmission proceeds.
In a paper entitled "A Hierarchical Data Structure for Picture Processing" published in Computer Graphics and Image Processing (Vol. 4, No. 2, pp. 104-119, June 1975) S. Tanimoto and T. Pavlidis have proposed for pattern recognition and data compaction purposes to process the digital representations of the pixels of highest resolution into progressively coarser levels of spatial resolution. The lower-resolution levels are coded by fewer bits of data and hence can be transmitted much faster than the final image. Picture analysis is said to be made more efficient by this type of image processing. The authors base their hierarchical structure on four-cell true averages.
Important applications of the hierarchical arrangement for image-processing data includes rapid remote browsing through a sequence of images. The recipient is thereby permitted to abort transmission of unwanted images as soon as they are recognizable, far short of complete transmission. Another application is teleconferencing by way of telephone lines where visual material, such as sketches and graphs, can be sent over auxiliary facsimile apparatus by presenting rough representations first and detail later. Other potential applications include interactive computer graphics and the sending of a picture from a transmitter threatened with imminent destruction, as in a fire or with seismographic alarm apparatus, where one hopes for the best possible picture in whatever time remains for transmission.